pre-processing has a single question of this writing. Is this tag useful?
I don't think this is a useful tag at all. Preprocessing is a type of solution, not a specific problem or solution per se. One is not an expert in "preprocessing things" in general, one is an expert at some particular task done with or during pre-processing, and in those cases the more specific tag related to that task should be used.
I think we should blacklist this tag.